Among substances that are oxygen-sensitive we would particularly mention beers (especially lager beers), wines (especially white ones), fruit juices, some carbonated soft drinks, fruits, nuts, vegetables, meat products, baby foods, coffee, sauces, and dairy products. Almost all foods and beverages are sensitive to some degree.
One approach to oxygen-sensitive products has been the inclusion in the pack of a sachet containing a compound such as iron or a lower iron oxide or hydroxide. This material reacts with ("scavenges") oxygen packed with the product or transmitted through the wall of the package.
Another approach has been the inclusion of scavenger in the walls of the package. Where the walls comprise a polymer and are appreciably oxygen-permeable this has the advantage of scavenging at least a part of the oxygen before it reaches the package contents at all.
Some discussion of the conventional measurements and units of oxygen permeation is appropriate at this point. The measurement is made by exposing a package wall of area A to a partial pressure p of oxygen on the one side and to an essentially zero partial pressure of oxygen on the other. The quantity of oxygen emerging on the latter side is measured and expressed as a volume rate dV/dt, the volume being converted to some standard conditions of temperature and pressure. After a certain time of exposure (usually a few days) dV/dt is generally found to stabilise, and a P.sub.W value is calculated from the equation (1). EQU dV/dt=P.sub.W Ap (1)
P.sub.W in the present specification and claims is called the permeance of the wall. (Analogy with magnetic permeance and electrical conductance would suggest that P.sub.W should be described as "permeance per unit area", but we are following the nomenclature in Encyclopaedia of Polymer Science and Technology, Vol. 2, Wiley Interscience, 1985, page 178.) The standard conditions for expressing dV/dt used generally and in this specification are 0.degree. C. and 1 atm (1 atm=101 325 N m.sup.-2). If the thickness of the area of wall is substantially constant over the area A with value T and the wall is uniform through the thickness (i.e. the wall is not a laminated or coated one) then the permeability of the material in the direction normal to the wall is calculated from the equation (2). EQU dV/dt=P.sub.M Ap/T (2)
For non-scavenging materials, P.sub.W and P.sub.M are to a reasonable approximation independent of t, p, and T although they are often appreciably dependent on other conditions of the measurement such as the humidity of the atmosphere on the oxygen-rich side and the temperature of the measurement.
For oxygen-scavenging walls, P.sub.W and P.sub.M are functions of t because the concentrations and activity of scavenger vary with time (particularly as the scavenger is consumed). This has not prevented us usually from measuring P.sub.W and P.sub.M reasonably accurately as a function of time (the changes in dV/dt being relatively gradual once the normal initial equilibration period of a few days is over). However, it should be recognised that, whereas after a few days' exposure to the measurement conditions a non-scavenging wall achieves a steady state in which dV/dt is equal to the rate of oxygen ingress to the wall, a scavenging wall achieves an (almost) steady state in which dV/dt is considerably less than the rate of oxygen ingress to the wall. This being the case, it is likely that P.sub.W calculated from (1) is a function of p as well as of t and that P.sub.M in (2) is a function of p and T as well as of t. P.sub.W and P.sub.M for scavenging walls are, strictly speaking, not true permeances and permeabilities at all (since permeation and scavenging are occurring simultaneously) but, rather, apparent ones. However, we have chosen to retain the conventional terms "permeance" and "permeability". So long as the conditions of the measurement are sufficiently specified they are suitable for characterising the walls in a manner relevant to the packaging user (i.e. in terms of the oxygen emerging from the wall).
All values of P.sub.W and P.sub.M hereinafter in this specification (except where stated otherwise) are to be understood to refer to conditions in which p=0.21 atm, the relative humidity on the oxygen-rich side of the wall is 50% and the temperature is 23.degree. C. Conditions close to these are conventional in the packaging industry.
It is possible for P.sub.W and P.sub.M to be affected by the illumination of the wall under test. All P.sub.W and P.sub.M values hereinafter, and indeed all references to oxidation, oxidisability, and oxygen-scavenging properties, refer to the dark or else to conditions of irradiation not appreciably contributing to oxygen-scavenging.
In our copending UK patent application 88 15699.7, we have described and claimed a wall for a package, which wall comprises, or includes a layer comprising, a composition comprising a polymer and having oxygen-scavenging properties, characterised in that the composition scavenges oxygen through the metal-catalysed oxidation of an oxidisable organic component thereof.
In a second aspect, the invention of the aforesaid UK patent application provides a composition for packaging use which comprises a polymer, an oxidisable organic component, and a metal catalyst for the oxidation of the oxidisable organic component.
The composition provided by the aforesaid invention has three major uses, namely as the material for a wall or a layer of a wall, as a masterbatch for blending with another polymer for such use, and as a head-space scavenger.
In a third aspect the aforesaid invention provides a package, whether rigid, semi-rigid, collapsible, lidded or flexible or a combination of these, a wall of which is a wall as provided by the invention in its first aspect or comprises entirely, as a layer, or as a blend the composition provided by the invention in its second aspect.
The UK patent application corresponds to EPC patent application 88 306175.6 and PCT patent application GB/8800532. The UK patent application has been published under number GB 2207439A and the EPC under number EP 301719 A1. The entire disclosure of the aforesaid patent applications is incorporated herein by this reference. However, it is convenient here to note the following points relating to our earlier invention:
(1) The oxidisable organic component may be an oxidisable polymer. The use of an oxidisable polymer as the oxidisable organic component has the advantage, broadly speaking, over the use of an oxidisable non-polymeric component that it is less likely to affect adversely the properties of a non-oxidisable polymer with which is is blended. It is possible for an oxidisable polymer to be used as the sole polymer in the composition, serving a dual function as polymer and oxidisable organic component.
(2) It is of course possible for two or more polymers, two or more oxidisable organic components, or two or more catalysts to be used. It is possible also for a metal catalyst to be used in combination with a non-metal catalyst.
(3) The word "catalyst" is used in a general way readily understood by the man skilled in the art, not necessarily to imply that it is not consumed at all in the oxidation. It is indeed possible that the catalyst may be converted cyclically from one state to another and back again as successive quantities of oxidisable component are consumed by successive quantities of oxygen. However, it may be that some is lost in side reactions, possibly contributing directly to oxygen-scavenging in small measure, or indeed that the "catalyst" is more properly described in an initiator (e.g. generating free radicals which through branching chain reactions lead to the scavenging of oxygen out of proportion to the quantity of "catalyst").
(4) Polyesters and polyolefins are especially suitable as non-oxidisable polymeric components, especially ethylene terephthalate or ethylene naphthalate polyesters. Oxidisable organic components include amides, especially polyamides and most especially MXD6, which is a condensation polymer of m-xylylenediamine and adipic acid. Metal catalysts include cobalt, copper, and rhodium compounds.
The aforesaid patent applications describe most particularly walls which would have a permeance in the range from 1.5, preferably 3.0 to 30, preferably 18.0 cm.sup.3 /(m.sup.2 atm day), in the absence of scavenging. Various multi-layer structures for walls are also described.